Each colour in white light is diffracted by a different angle which causes the colours to separate.
Light diffracts through a diffraction grating/film according to this equation:
d is the distance between slits in the grating/film,
n is the diffraction order.
White light is made up of the colours of the rainbow, each colour has its own wavelength. So when each colour is diffracted by the film for a given diffraction order (n) it will have a slightly different angle of diffraction (θ) than the other colours.
Red has the largest wavelength so it is diffracted most as you can see from the equation. Blue/violet has the shortest wavelength so it is diffracted least. Practically speaking this means that the blue end of the spectrum is nearer the centre line of the experiment and the red end is furthest from it.